1. Field of the Invention
This invention relates in general to a process for chemically bonding vapor deposited poly-p-xylylene to a thermoset resin and the article produced thereby. In one aspect, this invention relates to a process for chemically bonding poly-p-xylylenes to thermoset resins by pre-treatment of the poly-p-xylylenes with a low temperature plasma. In a further aspect, this invention is directed to poly-p-xylylenes coated with a thermoset resin wherein the bond between the two is chemical and superior to those currently available.
2. Description of the Prior Art
Vapor deposited para-xylylene polymers (parylenes) are commonly empoloyed to coat or encapsulate various types of substrates since they are insoluble in every common organic solvent at room temperature, are touch resistant, moisture resistant and exhibit low permeability to most gases and vapors. These polymers have been found to remain tough and flexible for a long period of time over a wide temperature range, thus permitting their use in conformally coating electronic assemblies such as printed circuit (PC) boards.
In some applications, it is sometimes necessary to repair or rework parylene coated electronic assemblies. This involves removing the parylene coating around a defective component and replacing the component. Then, a patching compound, typically a thermosetting resin, is applied around the new replacement component to protect it. The patching compound must adhere to the replacement component (the repaired region) and the unremoved parylene coating around the perimeter of this region. An important problem encountered in the commercial application of thermosetting resins has been the difficulty in their adhesion to the parylene surface to be coated. Attempts to improve adhesion between the thermoset resin and the unremoved parylene coating around the perimeter of the repaired region have involved mechanical roughening of the parylene with sandpaper or solvent swelling of the surface of the parylene.
It is well known that polymer films may be surface treated to improve adhesion and wettability through methods such as chemical treatment, flame exposure, beta-particle or gamma-ray bombardment, electrical corona exposure, glow discharge exposure or cold plasma exposure. The cold plasma approach has several advantages over the other methods: There is flexibility in choosing the interacting medium. A high degree of control over the conditions of the system can be exercised. And the treatment time is relatively short. Furthermore, the cold plasma treatment has a higher degree of safety than other techniques since chemical treatment usually involves strong acids or bases, flame treatment presents a heat problem, high energy radiation requires heavy shielding around the work area, and electrical corona requires electrode potential of several kilovolts. Most of these problems are absent with cold or low temperature plasmas, i.e., voltages are low, the radiation is negligible, and at the low operating pressure involving heat is not a major factor. In addition, the use of strong, corrosive chemicals is avoided.
However, prior art treatments of polymeric films by electrical discharge relied on chemical reactions or coatings on the surface which have a tendency to show up not only on the film surface but within the surface. In some instances, "treat-through" occurs so that a relatively thin film will be treated completely through its thickness and on both surfaces.
By "cold plasma" as used throughout this application is meant a discharge having a high electron temperature and a low gas temperature, a nonequilibrium system. The nonequilibrium condition can be explained by considering the mode of excitation. For example, when high frequency alternating current imposes polarity reversals upon the plasma particles in the order of several million times per second, the particles are alternately accelerated thereby greatly increasing the probability of particle collision. The collisions are sufficiently strong to ionize and disassociate the gas molecules and thereby produce a plasma which comprises chemically active ions, radical species and free electrons.
In a cold or low temperature plasma the pressure must be low, of the order of less than 100 torr, making possible a relatively large mean free path for the electrons produced by the excitation. As used herein, the unit "torr" is equivalent to one millimeter of mercury (1 mm Hg). Since electrons are lighter and far more mobile than the considerably heavier gas ions, energy is more readily and selectively imparted to the electrons. Thus, in a cold plasma, the gas temperature will be of the order of several hundred degrees kelvin while the electron energy will correspond to a temperature of several thousand degrees kelvin. Since this represents a low energy density when compared to "hot" plasmas such as those produced in an electric arc, the term "cold plasma" is applied. Another synonymous term is "glow discharge." The cold plasmas herein are generated at radiofrequencies just beyond audible frequencies and range from 15 thousand Hertz (Hz) per second up to and beyond 30 Megahertz (MHz) per second.